Surface treatment method

ABSTRACT

This invention relates to a surface treatment method comprising the steps of providing a treating solution containing a specific ammonium compound or aqueous ammonia, and magnesium silicofluoride, heating the treating solution to a temperature of 70 to 100° C., and soaking aluminum or an aluminum alloy in the treating solution; a piston having a surface coated with a film consisting of a mixture of NH 4 MgAlF 6  and MgAlF 5 .1.5H 2 O or a mixture of NH 4 MgAlF 6  and MgAl 2 F 8 .2H 2 O; and a sliding member made of aluminum or the like wherein the whole surface of the sliding member or the sliding surface thereof is coated with a film which consists of either of the aforesaid mixtures, has a cubic crystal structure, and shows no crystalline orientation.  
     The present invention can provide a surface treatment method which requires simple equipment, can reduce treatment costs, and can yield aluminum or an aluminum alloy having excellent abrasion resistance, corrosion resistance and other properties, as well as sliding members and pistons having excellent abrasion resistance and other properties.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a surface treatment method for aluminumor an aluminum alloy, and pistons surface-treated thereby, as well as asurface-treating film for aluminum or an aluminum alloy, and slidingmembers having a sliding surface coated therewith.

[0003] More particularly, this invention relates to a surface treatmentmethod which requires simple equipment, can reduce treatment costs, andcan yield aluminum or an aluminum alloy having excellent abrasionresistance, corrosion resistance and other properties, as well aspistons having undergone a surface treatment according to this method.It also relates to a surface-treating film suitable for use on thesliding surfaces of internal combustion engines and having excellentabrasion resistance, initial fitness, oil retention and otherproperties, and sliding members coated with such a slide film.

[0004] 2. Description of the Related Art

[0005] The Alumite treatment which has conventionally been employed is amethod for anodizing aluminum in an acid bath to form a hard aluminumoxide film on the aluminum surface. However, this method has thedisadvantage that it requires equipment for electric power supply andthat it involves a considerable cost because of a slow rate of filmformation.

[0006] On the other hand, the skirt of an aluminum or aluminum alloypiston as an internal combustion engine component is plated with tin.Although the deposited tin film is effective in bring about good initialfitness, it cannot be expected to have the effect of improving abrasionresistance.

SUMMARY OF THE INVENTION

[0007] In view of the above-described problems of conventional surfacetreatment techniques, the present inventors made intensiveinvestigations for the purpose of developing a surface treatment methodwhich requires simple equipment, can reduce treatment costs, and canform a uniform film having excellent corrosion resistance, abrasionresistance and other properties, as well as sliding memberssurface-treated by such a method.

[0008] As a result, the present inventors have found that theabove-described problems can be solved by a surface treatment methodcomprising the steps of providing a treating solution (or aqueoussolution) containing magnesium silicofluoride (MgSiF₆.6H₂O) and aspecific ammonium compound or the like, heating this treating solutionto a temperature in the range of 70 to 100° C., and soaking aluminum oran aluminum alloy in this treating solution.

[0009] Moreover, it has been found that the above-described problems canalso be solved by coating the whole surface of a sliding member or thesliding surface thereof with a specific film consisting of a mixture ofNH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O.

[0010] Furthermore, it has been found that the above-described problemscan also be solved, for example, by using a specific film formed on asurface of aluminum or an aluminum alloy, consisting of a mixture ofNH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O,and having silicon particles dispersed therein. The present inventionhas been completed from this point of view.

[0011] That is, according to a first aspect of the present invention,there is provided a surface treatment method comprising the steps ofproviding a treating solution containing an ammonium compound selectedfrom ammonium borofluoride and ammonium chloride, or aqueous ammonia,and magnesium silicofluoride (MgSiF₆.6H₂O ); heating the treatingsolution to a temperature of 70 to 100° C.; and soaking aluminum or analuminum alloy in the treating solution. In this surface treatmentmethod, the aforesaid treating solution preferably contains 0.1 to 20parts by weight of magnesium silicofluoride and 0.01 to 10 parts byweight, as expressed in terms of ammonium (NH₄), of the ammoniumcompound or aqueous ammonia. The ammonium compound used hereinpreferably comprises a compound which has a solubility of not less than1 g/L in water and, when dissolved, can provide ammonium ions (NH₄ ⁺) tothe solution. Specifically, ammonium borofluoride (NH₄BF₄) or ammoniumchloride (NH₄Cl) is used.

[0012] Moreover, the present invention also provides a piston havingundergone a surface treatment according to the above-described surfacetreatment method.

[0013] According to a second aspect of the present invention, there isprovided a piston wherein a surface thereof is coated with a filmconsisting of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture ofNH₄MgAlF₆ and MgAl₂F₈.2H₂O, and preferably its whole surface includingthe piston ring grooves, piston pin boss, skirt, piston head andinternal piston surface is coated with the film. In this piston, thethickness of the film is preferably in the range of 1 to 20 μm.

[0014] According to a third aspect of the present invention, there isprovided a sliding member made of a base metal comprising aluminum or analuminum alloy, wherein the whole surface of the sliding member or thesliding surface thereof is coated with a film which consists of amixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ andMgAl₂F₈.2H₂O, has a cubic crystal structure, and shows no crystallineorientation.

[0015] Moreover, there is also provided a sliding member wherein thewhole surface of the sliding member or the sliding surface thereof iscoated with a film which has a thickness of 1 to 20 μm and consists of aplurality of aggregates having a size of 1 to 20 μm, each aggregatebeing formed of microcrystals having a size of 1 μm or less.

[0016] According to a fourth aspect of the present invention, there isprovided a surface-treating film for aluminum alloys wherein thesurface-treating film is a film formed on a surface of aluminum or analuminum alloy, consisting of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂Oor a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O, and having silicon particlesdispersed therein, the content of silicon particles dispersed in thefilm is in the range of 1 to 24% by weight and preferably 6 to 24% byweight, and the content of silicon in the aluminum alloy is in the rangeof 4 to 24% by weight and 7 to 24% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic illustration of a piston in accordance withthe present invention;

[0018]FIG. 2 is a schematic view of a surface-treating film inaccordance with the present invention;

[0019]FIG. 3 is a schematic view of a surface-treating film inaccordance with the present invention;

[0020]FIG. 4 is a schematic view of a cross section Of a film;

[0021]FIG. 5 is an X-ray diffraction diagram of a film formed withtreating solution 1 of Example 1;

[0022]FIG. 6 is an X-ray diffraction diagram of a film formed withtreating solution 2 of Example 1;

[0023]FIG. 7 is a diagram showing the cross-sectional shapes of scratchmarks as observed in abrasion tests;

[0024]FIG. 8 is a graph showing wear volumes as observed in abrasiontests;

[0025]FIG. 9 is an electron micrograph of a film formed with treatingsolution 1 of Example 1;

[0026]FIG. 10 is an electron micrograph of a film formed with treatingsolution 2 of Example 1;

[0027]FIG. 11 is a photomicrograph of a cross section of a film formedwith treating solution 1 of Example 1;

[0028]FIG. 12 is a photomicrograph of a cross section of a film formedwith treating solution 2 of Example 1;

[0029]FIG. 13 is a graph showing the results of abrasion resistancetests carried out in Example 5;

[0030]FIG. 14 is an X-ray diffraction diagram of a film formed withtreating solution 3 of Example 6;

[0031]FIG. 15 is an X-ray diffraction diagram of a film formed withtreating solution 4 of Example 7; and

[0032]FIG. 16 is an X-ray diffraction diagram of a film formed withtreating solution 5 of Example 8.

[0033] The reference numerals given in these views are defined asfollows: 1, Piston; 2, Base metal; 3, Film; 4, Ring groove; 5, Skirt; 6,Pin hole; 7, Microcrystal; 8, Aggregate of microcrystals; 9, Silicon;10, Aluminum alloy; 11, Film on aluminum surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] First, the surface treatment method in accordance with the firstaspect of the present invention is described below.

[0035] The surface treatment method of the present invention comprisesthe steps of providing a treating solution (or aqueous solution)containing magnesium silicofluoride (MgSiF₆.6H₂O) and a specificammonium compound or aqueous ammonia, heating this treating solution toa temperature in the range of 70 to 100° C., and soaking aluminum or analuminum alloy in this treating solution.

[0036] The ammonium compound used in the present invention preferablycomprises a compound which has a solubility of not less than 1 g/L inwater and, when dissolved, can provide ammonium ions (NH₄ ⁺) to thesolution. Specifically, such compounds include ammonium borofluoride andammonium chloride. As the aqueous ammonia, there may be used aqueousammonia having an ordinary concentration.

[0037] The treating solution used in the present invention preferablycontains 0.1 to 20 parts by weight, more preferably 0.2 to 15 parts byweight, of magnesium silicofluoride (MgSiF₆.6H₂O) and 0.01 to 10 partsby weight as expressed in terms of ammonium (NH₄), more preferably 0.02to 5 parts by weight, of the aforesaid ammonium compound or the like,per 100 parts by weight of water. This treating solution makes itpossible to form a film having better uniformity, abrasion resistanceand corrosion resistance on the surface of the aluminum or aluminumalloy.

[0038] If the amount of magnesium silicofluoride (MgSiF₆.6H₂O) is lessthan 0.1 part by weight or the amount, as expressed in terms of ammonium(NH₄), of the ammonium compound or the like is less than 0.01 part byweight in the treating solution used in the present invention, thereaction will be retarded to extend the treating time to an undueextent.

[0039] On the other hand, if the amount of magnesium silicofluoride(MgSiF₆.6H₂O) is greater than 20 parts by weight or the amount, asexpressed in terms of ammonium (NH₄), of the ammonium compound or thelike is greater than 10 parts by weight, it may be difficult to dissolvethe compound.

[0040] The material to be surface-treated according to the presentinvention is aluminum or an aluminum alloy. Specific examples thereofinclude pure aluminum, flattened aluminum materials, cast aluminummaterials and die-cast aluminum materials. The present invention isapplicable to any of the foregoing materials and has the effect ofimproving abrasion resistance, corrosion resistance and other propertiesas a result of the surface treatment.

[0041] The pretreatment of a material to be surface-treated may becarried out simply by removing contaminants (e.g., oil) attachedthereto. However, its surface treatment may be carried out after thematerial is subjected to alkali etching with sodium hydroxide or thelike, and acid cleaning.

[0042] According to the present invention, the aluminum or aluminumalloy to be surface-treated is soaked in the aforesaid treating solution(or aqueous solution).

[0043] The temperature of the treating solution in which the aluminum oraluminum alloy is soaked is usually in the range of 70 to 100° C.,preferably 75 to 99° C., and more preferably 80 to 98° C. If thetemperature of the treating solution is lower than 70° C., the reactionwill be retarded to extend the treating time to an undue extent. On theother hand, if the temperature of the treating solution is higher than100° C., the evaporation of the treating solution will be increased toan undue extent.

[0044] As to the treating time, it is sufficient for surface-treatingpurposes to soak the material in the treating solution for a period ofabout 2 minutes, because the film-forming reaction is completed in aperiod of about 1 minute. It is to be understood that, once a film isformed, the material may be soaked in the treating solution for 30minutes or more without any problem, because this film has a protectiveeffect.

[0045] The surface-treating film formed on aluminum or the likeaccording to the above-described surface treatment method of the presentinvention has a protective effect and can hence improve the corrosionresistance of the aluminum base material. Moreover, the surface-treatingfilm so formed has excellent abrasion resistance.

[0046] On the other hand, since the surface treatment method of thepresent invention requires no equipment for electric power supply, theequipment can be simplified and this is very advantageous from theviewpoint of cost. Moreover, as compared with conventional surfacetreatment techniques, the surface treatment method of the presentinvention gives a faster rate of film formation on the surface ofaluminum or the like, and hence has higher productivity.

[0047] Next, the piston in accordance with the second aspect of thepresent invention is described below.

[0048] The piston of the present invention is a piston having undergonea surface treatment according to a surface treatment method whichcomprises the steps of providing a treating solution (or aqueoussolution) containing magnesium silicofluoride (MgSiF₆.6H₂O) and theaforesaid specific ammonium compound, heating this treating solution toa temperature in the range of 70 to 100° C., and soaking aluminum or analuminum alloy in this treating solution. In this method, it ispreferable that the aforesaid treating solution contain 0.1 to 20 partsby weight of magnesium silicofluoride (MgSiF₆.6H₂O) and 0.01 to 10 partsby weight, as expressed in terms of ammonium (NH₄), of the aforesaidammonium compound, per 100 parts by weight of water.

[0049] Before forming a film according to the above-described surfacetreatment method, the piston of the present invention is cleaned with anorganic solvent, a degreasing agent and the like. The present inventionmay be applied to a wide variety of common engine pistons made ofaluminum alloy.

[0050] The cleaned engine piston is soaked in the treating solutionaccording to the above-described surface treatment method. Thus, a filmconsisting of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture ofNH₄MgAlF₆ and MgAl₂F₈.2H₂O is formed on the piston surface. In thistreatment, a mixture of NH₄MgAlF₆ and MgAlF₆.1.5H₂O or a mixture ofNH₄MgAlF₆ and MgAl₂F₈.2H₂O is deposited in such a way that, when theamount of ammonium (NH₄) supplied is insufficient to form a filmconsisting of NH₄MgAlF₆ alone, MgAlF₅.1.5H₂O or MgAl₂F₈.2H₂Ocrystallized out in admixture with NH₄MgAlF₆. During this process, afilm consisting of NH₄MgAlF₆ alone may be partly formed. Either of theaforesaid films has excellent abrasion resistance.

[0051] As to the treating time, it is sufficient for surface-treatingpurposes to soak the piston in the treating solution for a period ofabout 2 minutes and preferably 2 to 10 minutes, because the film-formingreaction is completed in a period of about 1 minute similarly to theabove-described surface treatment method. It is to be understood that,once a film is formed, the piston may be soaked in the treating solutionfor 30 minutes or more without any problem, because this film has aprotective effect.

[0052] In the piston of the present invention which is obtained in theabove-described manner, its surface is coated with a film consisting ofa mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ andMgAl₂F₈.2H₂O, and optionally including a film consisting of NH₄MgAlF₆alone, and hence exhibits excellent surface properties. This film canproduce a beneficial effect even if it is formed on any of various partssuch as the piston ring grooves, piston pin boss, skirt, piston head andall side faces. However, it is preferable that its whole surfaceincluding these parts be coated with the film.

[0053] The above-described piston of the present invention is not sosoft as conventional pistons plated, for example, with tin, but hasexcellent abrasion resistance and very good durability.

[0054] Next, the sliding member in accordance with the third aspect ofthe present invention is described below.

[0055] The sliding member of the present invention is made of a basemetal comprising aluminum or an aluminum alloy, and the whole surface ofthe sliding member or the sliding surface thereof is coated with a filmwhich consists of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixtureof NH₄MgAlF₆ and MgAl₂F₈.2H₂O, has a cubic crystal structure, and showsno crystalline orientation. Either of the aforesaid films has excellentabrasion resistance, and may include a film consisting of NH₄MgAlF₆alone.

[0056] Moreover, the sliding member of the present invention is suchthat the whole surface of the sliding member or the sliding surfacethereof is coated with a film which has a thickness of 1 to 20 μm andconsists of a plurality of aggregates having a size of 1 to 20 μm, eachaggregate being formed of microcrystals having a size of 1 μm or less.

[0057] This sliding member is more specifically described below withreference to FIG. 1.

[0058] Referring to FIG. 1, a piston 1 for use in an internal combustionengine, which is a sliding member, is made of a base metal 2 comprisingan aluminum alloy. The surface of the base metal is coated with a film 3for improving its sliding characteristics. Its skirt 5 slides over theinner wall of a cylinder bore constituting the opposite member, its ringgrooves 4 slide against piston rings, and its pin hole 6 slides over apiston pin.

[0059] By way of example, an Al—Si—Cu—Ni—Mg alloy or the like is used asbase metal 2. Specific examples of the alloy include AC8A, AC8B, AC9Aand AC9B.

[0060] Film 3 is formed on the surface of piston 1 by subjecting piston1 to a chemical conversion treatment. This film 3 may comprise a filmconsisting of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture ofNH₄MgAlF₆ and MgAl₂F₈.2H₂O. Optionally, this film 3 may include a filmconsisting of NH₄MgAlF₆ alone. Even though film 3 has any of thesecompositions, it has a cubic crystal structure and shows no crystallineorientation.

[0061] This film 3 consists of microcrystals 7 having a size of 1 μm orless. These microcrystals 7 gather together to form a plurality ofaggregates 8 having a size of 1 to 20 μm, and these aggregates 8 coatthe surface of the base metal to a thickness of 1 to 20 μm (FIGS. 2 and3). This film forms a new sliding surface. The configuration of theseaggregates 8 varies according to the amount of MgAlF₅.1.5H₂O or MgAl₂F₈.2H₂O mixed with NH₄MgAlF₆.

[0062] These microcrystals 7 and aggregates 8 constituting film 3 causesan increase in the surface area of the sliding surface and hence animprovement in oil retention. Moreover, since aggregates 8 arepreferentially worn away, the sliding surface exhibits good initialfitness. These improvements in wearing characteristics are effective inenhancing the durability of the sliding member, reducing friction, andimproving fuel consumption.

[0063] Finally, the surface-treating film in accordance with the fourthaspect of the present invention is described below.

[0064] The surface-treating film for aluminum alloys in accordance withthe present invention is a film formed on a surface of aluminum or analuminum alloy, consisting of a mixture of NH₄MgAlF₆ and MgAlF₅.1.5H₂Oor a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O, and having silicon particlesdispersed therein. Optionally, this film may include a film consistingof NH₄MgAlF₆ alone. Moreover, the content of silicon particles dispersedin the film is in the range of 1 to 24% by weight and preferably 6 to24% by weight, and the content of silicon in the aforesaid aluminumalloy is in the range of 4 to 24% by weight and preferably 7 to 24% byweight.

[0065] The film structure obtained according to the present invention isshown in FIG. 4. The aluminum alloy constituting the base metal contains4 to 24% by weight of silicon 9, and eutectic silicon or proeutecticsilicon is dispersed in the aluminum matrix 10. The surface of thealuminum alloy is coated with a film consisting of a mixture ofNH₄MgAlF₆ and MgAlF₅1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O,and silicon particles similar to the eutectic silicon or proeutecticsilicon dispersed in the aluminum alloy base metal are dispersed in thisfilm.

[0066] The above-described structure of the surface-treating film inaccordance with the present invention can be obtained in the followingmanner.

[0067] An aluminum alloy material containing 4 to 24% by weight ofsilicon (Si) is degreased with an organic acid, a degreasing agent orthe like, and then subjected to alkali etching with sodium hydroxide orthe like, and acid cleaning. Subsequently, a treating solution (oraqueous solution) containing magnesium silicofluoride (MgSiF₆.6H₂O) andan ammonium compound is heated to a temperature in the range of 70 to100° C., and the aluminum alloy material is soaked therein for a periodof about 2 to 10 minutes.

[0068] According to the above-described procedure, aluminum present inthe surface of the aluminum alloy material is preferentially dissolvedin the treating solution. At the same time, the dissolved aluminum (Al)reacts with fluorine (F), magnesium (Mg) and ammonium (NH₄) present inthe solution to form a film consisting of a mixture of NH₄MgAlF₆ andMgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O. This filmdeposits on the aluminum alloy surface while incorporating siliconparticles which are hard to dissolve in the treating solution. Thus,there is obtained a film having silicon dispersed therein.

[0069] However, it is to be understood that, in the above-describedprocedure, degreasing with an organic solvent, degreasing agent or thelike, alkali etching and acid cleaning serve to clean the base materialand are not directly required to obtain the film structure of thepresent invention.

[0070] The present invention is more specifically explained withreference to the following examples. However, these examples are not tobe construed to limit the scope of the present invention.

[0071] The surface treatment method for aluminum or an aluminum alloy inaccordance with the present invention requires simple equipment, canreduce treatment costs, and can yield a film having excellent abrasionresistance, corrosion resistance and other properties.

[0072] That is, according to the present invention, the equipment can besimplified because the treating conditions are easy, and the resultingsurface-coated aluminum or the like has excellent abrasion resistanceand can reduce friction losses. Moreover, the film obtained by themethod of the present invention has protective properties, so that ithas a uniform film thickness over the whole surface of aluminum or thelike without regard to the treating conditions, and shows littleinequality in film thickness. Furthermore, the film thus obtained hasexcellent corrosion resistance and hence exhibits abrasion resistanceeven in a corrosive environment.

[0073] Moreover, the pistons having undergone a surface treatmentaccording to the method of the present invention have excellent abrasionresistance, corrosion resistance and other properties. Accordingly, theyhave excellent durability and can be effectively used as pistons forvarious engines.

[0074] Furthermore, the durability of sliding members of engines,compressors and the like can be improved by coating the sliding surfacesof the sliding members made of aluminum or an aluminum alloy accordingto the present invention. For example, if engine pistons are coated,improvements in abrasion resistance, initial fitness, oil retention andother properties can be achieved. This is effective in enhancingdurability, reducing friction, improving engine output, and improvingfuel consumption, and hence has a very important significance from anindustrial point of view.

EXAMPLE 1

[0075] This is an example in which an aluminum alloy was surface-treatedwith treating solutions (or aqueous solutions) containing magnesiumsilicofluoride (MgSiF₆.6H₂O) and ammonium silicofluoride [(NH₄)₂SiF₆].

[0076] 0.67 part by weight of magnesium silicofluoride (MgSiF₆.6H₂O) and0.13 part by weight of ammonium silicofluoride [(NH₄)₂SiF₆] weredissolved in 100 parts by weight of water. This aqueous solution washeated to 90° C. and used as treating solution 1. An AC8A-T6 castaluminum specimen having a diameter of 50 mm and a thickness of 5 mm wascleaned with an organic solvent and a degreasing agent, and thensurface-treated by soaking it in this treating solution 1 for 5 minutes.

[0077] Next, 0.67 part by weight of magnesium silicofluoride(MgSiF₆.6H₂O) and 0.33 part by weight of ammonium silicofluoride[(NH₄)₂SiF₆] were dissolved in 100 parts by weight of water. Thisaqueous solution was heated to 90° C. and used as treating solution 2.An AC8A-T6 cast aluminum specimen having a diameter of 50 mm and athickness of 5 mm was cleaned with an organic solvent and a degreasingagent, and then surface-treated by soaking it in this treating solution2 for 5 minutes.

[0078]FIG. 5 shows the results of analysis of the formed film with anX-ray diffractometer, for the specimen having been surface-treated withthe aforesaid treating solution 1.

[0079] It can be seen from FIG. 5 that a film consisting of a mixture ofNH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂Owas formed on the surface of the cast aluminum specimen as a result ofthe treatment with treating solution 1. However, this X-ray diffractiondiagram includes both the X-ray diffraction spectrum of the formed filmand the X-ray diffraction spectra of aluminum (Al) constituting the basemetal (i.e., AC8A-T6 material) and silicon (Si) contained in the basemetal. It can also be seen from this X-ray diffraction diagram that theformed film showed no crystalline orientation.

[0080]FIG. 6 shows the results of analysis of the formed film with anX-ray diffractometer, for the specimen having been surface-treated withthe aforesaid treating solution 2.

[0081] It can be seen from FIG. 6 that a film of NH₄MgAlF₆ was formed onthe surface of the cast aluminum specimen as a result of the treatmentwith treating solution 2. However, this X-ray diffraction diagramincludes both the X-ray diffraction spectrum of the formed film and theX-ray diffraction spectra of aluminum (Al) constituting the base metal(i.e., AC8A-T6 material) and silicon (Si) contained in the base metal.It can also be seen from this X-ray diffraction diagram that the formedfilm showed no crystalline orientation.

[0082] With respect to the specimens having undergone the surfacetreatment of the present invention using the aforesaid treating solution1 or 2, ball-on-disk abrasion tests were performed by using heat-treatedSCM435 material as the opposite material. Moreover, a specimen (ofAC8A-T6 material) having undergone no surface treatment was alsosubjected to a ball-on-disk abrasion test in the same manner.

[0083] The cross-sectional shapes (or profiles) of the scratch marks soproduced is shown in FIG. 7.

[0084] The wear volumes obtained from the results of the ball-on-diskabrasion tests are shown in FIG. 8.

[0085] It can be seen from FIG. 7 that, when compared with the specimenhaving undergone no surface treatment, the specimens having undergonethe surface treatment of the present invention using the aforesaidtreating solution 1 or 2 had excellent abrasion resistance.

[0086] It can be seen from FIG. 8 that the specimens having undergonethe surface treatment of the present invention using the aforesaidtreating solution 1 or 2 showed a wear volume equal to about {fraction(1/20)} of that of the specimen having undergone no surface treatment,and hence had excellent abrasion resistance.

[0087] Moreover, it can be seen that, whether the film formed as aresult of the surface treatment consisted of NH₄MgAlF₆ alone, a mixtureof NH₄MgAlF₆ and MgAlF₅.1.5H₂O, or a mixture of NH₄MgAlF₆ andMgAl₂F₈.2H₂O, the specimens having undergone the surface treatmentshowed a marked improvement in abrasion resistance as compared with thespecimen having undergone no surface treatment.

[0088]FIG. 9 is an electron micrograph of the formed film, for thespecimen having undergone the surface treatment using the aforesaidtreating solution 1. FIG. 2 is a schematic view corresponding to FIG. 9.

[0089] It can be seen from FIGS. 2 and 9 that the film consists ofmicrocrystals 7 having a size of 1 μm or less and these microcrystalsgather together to form a plurality of aggregates 8 having a size of 1to 20 μm. Moreover, it can be seen that the plurality of aggregates 8covers the surface of the base metal (i.e., AC8A-T6 material) to form afilm.

[0090]FIG. 10 is an electron micrograph of the formed film, for thespecimen having undergone the surface treatment using the aforesaidtreating solution 2. FIG. 3 is a schematic view corresponding to FIG.10.

[0091] It can be seen from FIGS. 3 and 10 that the film consists ofmicrocrystals 7 having a size of 1 μm or less and these microcrystalsgather together to form a plurality of aggregates 8 having a size of 1to 20 μm. Moreover, it can be seen that the plurality of aggregates 8covers the surface of the base metal (i.e., AC8A-T6 material) to form afilm.

[0092] It can also be seen from FIGS. 9 and 10 that the configuration ofaggregates 8 varies according to the composition of the film, i.e., theamount of MgAlF₅.1.5H₂O or MgAl₂F₈2H₂O mixed with NH₄MgAlF₆ (see FIGS. 2and 3).

[0093]FIGS. 11 and 12 are photomicrographs of a cross section of theformed film, for the specimens having undergone the surface treatmentusing the aforesaid treating solution 1 or 2. FIG. 4 is a schematic viewcorresponding to FIGS. 11 and 12.

[0094] It can be seen from FIGS. 4, 11 and 12 that a film having athickness of about 6 μm was formed on the surface of the base metal(i.e., AC8A-T6 material). Moreover, it can be seen that silicon (Si)particles derived from the base metal (i.e., AC8A material) areincorporated in the films shown in these photomicrographs. This is dueto the fact that silicon particles contained in the base metal (i.e.,AC8A-T6 material) remained on the surface of the base metal and wereincorporated into the film formed during the surface treatment process.

EXAMPLE 2

[0095] An AC8A-T6 cast aluminum specimen having a diameter of 50 mm anda thickness of 5 mm was cleaned with an organic solvent and a degreasingagent, and then surface-treated by soaking it in a treating solutionsimilar to treating solution 2 of the above Example 1 for 5 minutes.

[0096] With respect to the specimen having undergone the above-describedsurface treatment (i.e., the specimen of Example 2) and a specimen nothaving undergone the above-described surface treatment (i.e., anuntreated specimen), their corrosion resistance was evaluated by saltwater spray tests.

[0097] It can be seen from the results thus obtained that the specimenof the present invention showed a protective effect and thesurface-treating film formed by the above-described surface treatmentmethod could improve the corrosion resistance of aluminum or an aluminumalloy.

EXAMPLE 3

[0098] An AC8A-T6 cast aluminum specimen having a diameter of 50 mm anda thickness of 5 mm was cleaned with an organic solvent and a degreasingagent, and then surface-treated by soaking it in a treating solutionsimilar to treating solution 2 of the above Example 1 for 5 minutes.

[0099] With respect to the specimen having undergone the above-describedsurface treatment (i.e., the specimen of Example 3), a specimen (ofAC8A-T6 material) having undergone a surface treatment with hardAlumite, and a specimen (of AC8A-T6 material) having undergone nosurface treatment, their coefficients of friction under oil lubricationwere measured by using SCM material as the opposite material. Theresults thus obtained are shown in Table 1. TABLE 1 Coefficient offriction Example 3 0.09 Hard Alumite treatment 0.13 Untreated specimen0.12

[0100] It can be seen from the results shown in Table 1 that theabove-described surface treatment caused a reduction in coefficient offriction.

EXAMPLE 4

[0101] An engine piston (made of AC8A-T6 material) was cleaned with anorganic solvent and a degreasing agent, and then surface-treated bysoaking it in a treating solution similar to treating solution 2 of theabove Example 1 for 5 minutes. Thus, a film of NH₄MgAlF₆ was formed onthe surface of the piston.

[0102] Both the piston having undergone the above-described surfacetreatment of the present invention (i.e., the piston of Example 4) and apiston not having undergone the above-described surface treatment (i.e.,an untreated piston) were assembled into an engine, and this engine wasactually operated at full load.

[0103] After operation, each piston was removed and inspected forsurface conditions. The inspection items included adhesion of aluminumto the rings, scoring of the pin boss surface, and scoring of the skirtsurface. The results thus obtained are shown in Table 2. TABLE 2 Pistonof Example 4 Untreated piston Adhesion of Al to No Yes rings Scoring ofpin boss No Yes surface Scoring of skirt No Yes surface

[0104] It can be seen from the results shown in Table 1 that the pistonhaving undergone the surface treatment of the present invention showedimprovements over the untreated piston with respect to all of the pistonring grooves, pin boss and skirt surface.

EXAMPLE 5

[0105] Six aluminum alloy materials having different silicon (Si)contents were cleaned with an organic solvent and a degreasing agent,and then surface-treated by soaking it in a treating solution similar totreating solution 2 of the above Example 1 for 5 minutes. As a result,aluminum (Al) in the aluminum alloy materials was dissolved in thetreating solution. At the same time, the dissolved aluminum (Al) reactedwith fluorine (F), magnesium (Mg) and ammonium (NH₄) in the treatingsolution to deposit a film on the aluminum alloy surface. During thisprocess, the aforesaid film was formed while incorporating thereintosilicon particles which were hardly soluble in the treating solution, sothat a film having silicon dispersed therein was obtained. Thus, variousspecimens coated with films having different silicon (Si) contents wereobtained.

[0106] The specimens thus obtained were subjected to pin-on-discabrasion tests in which a cementation-hardened and tempered SCM420 pinwas used as the opposite material. In these tests, the abrasionresistance of the films was evaluated in terms of wear volume. Theresults thus obtained are shown in FIG. 13.

[0107] It can be seen from the results that, when compared with thespecimen containing no silicon (Si) in the film, even the specimenhaving a silicon content of about 1% showed an improvement in abrasionresistance, and that the specimens having a silicon content of 6% orgreater were found to show a marked improvement in abrasion resistance.

EXAMPLE 6

[0108] This is an example in which an aluminum alloy was surface-treatedwith a treating solution (or aqueous solution) containing magnesiumsilicofluoride (MgSiF₆.6H₂O) and ammonium borofluoride (NH₄BF₄).

[0109] 0.67 part by weight of magnesium silicofluoride (MgSiF₆.6H₂O) and0.51 part by weight of ammonium borofluoride (NH₄BF₄) were dissolved in100 parts by weight of water. This aqueous solution was heated to 90° C.and used as treating solution 3. An AC8A-T6 cast aluminum specimenhaving a diameter of 50 mm and a thickness of 5 mm was cleaned with anorganic solvent and a degreasing agent, and then surface-treated bysoaking it in the above treating solution for 5 minutes.

[0110]FIG. 14 shows the results of analysis of the formed film with anX-ray diffractometer, for the specimen having been surface-treated withthe aforesaid treating solution 3 according to the present invention.

[0111] It can be seen from FIG. 14 that a film of NH₄MgAlF₆ was formedon the surface of the cast aluminum specimen as a result of thetreatment with treating solution 3. However, this X-ray diffractiondiagram includes both the X-ray diffraction spectrum of the formed filmand the X-ray diffraction spectra of aluminum (Al) constituting the basemetal (i.e., AC8A-T6 material) and silicon (Si) contained in the basemetal.

EXAMPLE 7

[0112] This is an example in which an aluminum alloy was surface-treatedwith a treating solution (or aqueous solution) containing magnesiumsilicofluoride (MgSiF₆.6H₂O) and ammonium chloride (NH₄Cl).

[0113] 0.67 part by weight of magnesium silicofluoride (MgSiF₆.6H₂O) and0.26 part by weight of ammonium chloride (NH₄Cl) were dissolved in 100parts by weight of water. This aqueous solution was heated to 90° C. andused as treating solution 4. An AC8A-T6 cast aluminum specimen having adiameter of 50 mm and a thickness of 5 mm was cleaned with an organicsolvent and a degreasing agent, and then surface-treated by soaking itin the above treating solution for 5 minutes.

[0114]FIG. 15 shows the results of analysis of the formed film with anX-ray diffractometer, for the specimen having been surface-treated withthe aforesaid treating solution 4 according to the present invention.

[0115] It can be seen from FIG. 15 that a film of NH₄MgAlF₆ was formedon the surface of the cast aluminum specimen as a result of thetreatment with treating solution 4. However, this X-ray diffractiondiagram includes both the X-ray diffraction spectrum of the formed filmand the X-ray diffraction spectra of aluminum (Al) constituting the basemetal (i.e., AC8A-T6 material) and silicon (Si) contained in the basemetal.

EXAMPLE 8

[0116] This is an example in which an aluminum alloy was surface-treatedwith a treating solution (or aqueous solution) containing magnesiumsilicofluoride (MgSiF₆.6H₂O) and an aqueous solution of ammonia.

[0117] 0.67 part by weight of magnesium silicofluoride (MgSiF₆.6H₂O) and2 ml of aqueous ammonia (25% aqueous solution) were dissolved in 100parts by weight of water. This aqueous solution was heated to 90° C. andused as treating solution 5. An AC8A-T6 cast aluminum specimen having adiameter of 50 mm and a thickness of 5 mm was cleaned with an organicsolvent and a degreasing agent, and then surface-treated by soaking itin the above treating solution for 5 minutes.

[0118]FIG. 16 shows the results of analysis of the formed film with anX-ray diffractometer, for the specimen having been surface-treated withthe aforesaid treating solution 5 according to the present invention.

[0119] It can be seen from FIG. 16 that a film consisting of a mixtureof NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ andMgAl₂F₈.2H₂O was formed on the surface of the cast aluminum specimen asa result of the treatment with treating solution 5. However, this X-raydiffraction diagram includes both the X-ray diffraction spectrum of theformed film and the X-ray diffraction spectra of aluminum (Al)constituting the base metal (i.e., AC8A-T6 material) and silicon (Si)contained in the base metal.

1. A surface treatment method comprising the steps of providing atreating solution containing an ammonium compound selected from ammoniumborofluoride and ammonium chloride, or aqueous ammonia, and magnesiumsilicofluoride; heating the treating solution to a temperature of 70 to100° C.; and soaking aluminum or an aluminum alloy in the treatingsolution.
 2. A surface treatment method as claimed in claim 1 whereinthe treating solution contains 0.1 to 20 parts by weight of magnesiumsilicofluoride and 0.01 to 10 parts by weight, as expressed in terms ofammonium, of the ammonium compound or aqueous ammonia, per 100 parts byweight of water.
 3. A piston having undergone a surface treatmentaccording to a surface treatment method as claimed in claim 1 or
 2. 4. Apiston having a surface coated with a film consisting of a mixture ofNH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O.5. A piston as claimed in claim 4 wherein its whole surface includingthe piston ring grooves, piston pin boss, skirt, piston head andinternal piston surface is coated with the film consisting of a mixtureof NH₄MgAlF₆ and MgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ andMgAl₂F₈.2H₂O.
 6. A piston as claimed in claim 4 or 5 wherein thethickness of the film consisting of a mixture of NH₄MgAlF₆ andMgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O is in the rangeof 1 to 20 μm.
 7. A sliding member made of a base metal comprisingaluminum or an aluminum alloy, wherein the whole surface of the slidingmember or the sliding surface thereof is coated with a film whichconsists of a mixture of NH₄MgAlF₆ and MgAlF₅1.5H₂O or a mixture ofNH₄MgAlF₆ and MgAl₂F₈.2H₂O, has a cubic crystal structure, and shows nocrystalline orientation.
 8. A surface-treating film for aluminum alloyswherein the surface-treating film is a film formed on a surface ofaluminum or an aluminum alloy, consisting of a mixture of NH₄MgAlF₆ andMgAlF₅.1.5H₂O or a mixture of NH₄MgAlF₆ and MgAl₂F₈.2H₂O, and havingsilicon particles dispersed therein, the content of silicon particlesdispersed in the film is in the range of 1 to 24% by weight, and thecontent of silicon in the aluminum alloy is in the range of 4 to 24% byweight.